Hardened lava

. In the latest product still active volcanoes can not expect them to familiarize themselves with these entities, we must turn to ancient volcanic rocks, which, due to erosion, weathering and other destructive processes, bared their deepest parts. Especially curious columnar separately occurring commonly in the basalts and rarer in the porphyries, trachytes and diabase. As an exception Iddings observed the same individual in obsidian North American national park on the river Ielloustone. The value of these columns is different, and sometimes they reach 100 - 150 p. long and 8 - 9 inches in width, on the contrary, in the mountain Baula in Iceland some columns are as thick as a finger, while others reach up to ? 9. in thickness. The reason for such remarkable structures is the reduction of the lava when it cools. Due to cracking of the mass usually splits the multifaceted and often in hexagonal columns, deep in the stream, they are perpendicular to the surface in the upper parts or no decomposition occurs in isolation, or the formation of irregular prisms, grouped in a mess, and placed perpendicular to the main cracks. Sometimes the tops of these flows represent thin-bedded isolation even in the form of hexagonal bars and plates. The boundary between the upper and lower part of the act so suddenly that at first glance it seems as if moved here two streams, one above the other. But a closer study of one or the other side is so similar that their belonging to one thread becomes unquestionable. A curious example of this kind is Fingal's Cave on the island of Staffa off the coast of Scotland. The whole mass of basalt is part of the damaged stream, the lower part of which collapsed on the beautiful expression of the pillars, the upper also represent a solid mass. A perfect example of columnar basalt cover separately is Fig. 157, representing a district on the banks of the Colorado River, in North America. Lava flows once filled this area, new deposits have covered it, and then the river bed was dug themselves in basalts.