1052 Marvels of the Untverse 
of moths, and the young and adult forms of numerous other insects. A mass of Oak-apples kept 
under observation produced no less than seventy-five different species of insects, the total number of 
specimens obtained being upwards of fifty-five thousand. We perceive, therefore, that an Oak- 
apple is not merely a beautiful natural object, but a populous centre of insect activity. 
SLOW INOCIKS AVRIE IBWIOL I 
Nature has fashioned the rocks which form the crust of the earth in a great variety of ways, and 
has drawn upon a great number of materials for their composition. The old geologists considered 
that the first crust which formed on the surface of our molten earth was granite, but this has been 
Photo by] [Harold Bastin. 
ROOT-GALLS. 
These underground galls do not appear to have connection with Oak-apples, but it is from these that the ‘winter 
females ’’ emerge whose egg-laying produces the “apples’’ on the oak-twigs. 
proved a mere speculation, and the study of petrology brings it home to us that the crust has been 
built up in many stages by the aid of fire, water and wind. Chemical forces have also been at work, 
since most of the elements present in rocks are combined with one another to form comparatively 
few common minerals, which may be recognized without artificial aid when large enough. The 
microscope can, however, often tell more than a complete chemical analysis, because it shows us 
the internal method of arrangement of the minerals present, and this is of the utmost service in 
estimating the value of the rock, and also in forming a fairly accurate surmise of the life-history 
of the formation. 
The fascinating study of the microscopic structure of crystals as indicating the origin of minerals 
and rocks was first put on a practical basis in this country by Dr. Sorby, of Sheffield, fully fifty years 
ago, and he employed thin sections which had been polished down until they were transparent. 
Although the portion under the microscope is very small, yet such investigation affords most valuable 
help in understanding how the rock itself came into being. It can indicate the order in which the 
