96 MICROSCOPIC OBJECTS. 



called the Ketton oolite, is a beautiful object for the mi- 

 croscope. For building purposes those sorts in which 

 the grains are smallest are preferred, as the Portland 

 and Caen, the former of which was employed in the 

 erection of Saint Paul's Cathedral, and the latter for the 

 internal parts of the new Houses of Parliament. Some 

 oohtes are called shelly, from the number of shells and 

 fragments which enter into their composition. In general 

 these stones are not very durable, but for the microscope 

 are especially interesting. The principal localities for 

 obtaining them are Bamack, near Stamford ; Box, near 

 Bath ; and Cranmore, near Doulting, in Wiltshire. In 

 the oolites from the Trade quarries and the Grove quarry- 

 Bowers, at Portland, the shells are abundant. 



Limestones are usually very fine-grained, and there- 

 fore not so generally valuable for the microscope as 

 oolites. A collection of thin sections of these stones 

 mounted as transparent bodies, together mth two speci- 

 mens of each, (the one with a fractured and the other a 

 polished surface,) mounted as opaque objects, wiU convey 

 to the observer much valuable information on the geolo- 

 gical structure of these rocks. — Magnifying power 25 to 

 60 diameters. 



Sand. — The sand (Calcaire grossierej found at Grig- 

 non, near Paris, contains an abundant variety of fossil 

 shells, many of which are both curious and instmctive 

 microscopic objects. They are best examined as opaque 

 objects with a silver concave reflector. 



Sand, Bahama. — The grains of this sand are of a globu- 



