Geology and Mineralogy. 405 



Each layer of the clay, on an average of two-fifths of an inch 

 thick, represents a year's deposit. The clays are, at the North- 

 ampton bridge, above 120 feet thick, and at East Street bridge 

 above fifty feet, which would give numbers for the duration of 

 the lake favoring the idea that the Glacial period was not more 

 than 10,000 years ago, one of the shortest estimates. 



In these clays I have found an abundant glacial flora, proving 

 that the lake succeeded immediately to the ice, and I have found 

 indications of several re-advances of the ice ploughing up the 

 sands of the lake." 



4. Geological work of Marmots. — Professor Mushketoff's 

 account of his explorations in the Caspian steppes contains some 

 interesting remarks on the work done by marmots {Spermophylus 

 Eversmanii) in the modification of the surface of the steppe. They 

 made their appearance in the region only a few years ago, but 

 their heaps of earth already cover hundreds of square miles. Like 

 earthworms, they must therefore be regarded as a factor of some 

 importance in modifying the surface of the soil. Their heaps of 

 earth have an average length of 3£ meters, and a width of 2^ 

 meters, with an average height of from 30 to .50 centimeters, and 

 it was found that on each 2 square meters there were no less than 

 five, seven or even eight heaps, each of which represented at least 

 2 cubic meters of earth removed. It may be safely asserted that 

 on each square kilometer of surface no less than 30,000 cubic 

 meters of earth have been brought to' the surface owing to their 

 activity. Their influence on vegetation is also well worthy of 

 notice. — Nature, Oct. 6, p. 541. 



5. Slide at Lake Zug of July, 1887. — On July 5, 1887, at the 

 town of Zug, in Switzerland, a portion of the shore gave way and 

 sank into the lake. About three hours later another much larger 

 adjacent area also suddenly subsided, so that in all an area con- 

 siderably over two acres, with half of one of the principal streets, 

 was submerged to a depth of about 20 feet. It can be seen that 

 the subsoil consists of coarse gravel and sand, followed after a few 

 feet by soft wet sand and tine mud. According to Professor 

 Heim, this fine mud or sludge reaches to a depth of nearly 200 

 feet, and the disaster is shown to be due to a flowing out into the 

 lake of this mobile sludge from under the superincumbent weight 

 of buildings and firmer ground. The buildings collapsed as they 

 sank. . The catastrophe must have been long impending ; the ex- 

 act cause which precipitated it is undetermined, but a low level 

 of the lake and tremors from pile-driving for new quays are sug- 

 gested as contributories. — Geol. Mag., Oct., 1887. 



6. Organic origin of Chert. — Dr. G. J. Hinde has a paper in 

 the Geological Magazine, for October, showing conclusively that 

 the chert from the Carboniferous limestones of Ireland was all 

 made chiefly from the siliceous spicules of sponges, and that the 

 silica of silicified fossils has the same source. 



7. A sketch of Geological History, being the Natural History 

 of the Earth and of its prehuman inhabitants-, by Edward 



