Burbauk.] 196 [AprU 19. 



those that I have examined. At one of the quarries on Bobbins' 

 Hill, in Chelmsford, it can be seen that the limestone rested against 

 the irregularly fractured ends of strata of gneiss, which fill a small 

 space in one of the excavations, completely dividing the limestone 

 into two masses. A similar appearance is less plainly shown at (e) 

 in the sketch given. 



I propose next to present some facts in regard to the mineralogical 

 character and relative position, in these deposits, of the various mate- 

 rials of which they are composed. Some of these facts and conclu- 

 sions cannot readily be verified by examining the exhausted quarries 

 at Chelmsford, but have been derived from observations made, and 

 specimens collected, several years ago at Bolton, while the work of 

 excavation was still in progress. 



The central and principal part of the mass which filled the veins 

 and pockets and constituted the bulk of the deposit, was a coarsely 

 crystalline magnesian limestone, homogeneous in structure, and showing 

 no traces of stratification. In examining numerous specimens of this 

 limestone from the different quarries, I have found in it no traces of 

 the eozoonal structure. 



The various silicates which form the large number of distinct min- 

 erals for which these localities are noted, occur only attached to, or 

 near, the enclosing walls of the cavities, and generally in bands or lay- 

 ers, though sometimes irregularly distributed. They are found gen- 

 erally in pretty regular succession. A network of interlacing crystals 

 of actinolite, with smoky quartz, calcite and phlogopite, may be seen 

 attached to the walls; and passing inward there are found pyroxene, 

 scapolite, apatite, boltonite, fine fibrous tremolite, etc.; and also green 

 serpentine in irregular bands or layers, traversed by narrow seams of 

 chrysotile ; or scattered through the rock in irregular rounded grains 

 and masses, with intervening spaces filled with calcite. 



In these portions of calcite are found the radiating and branching 

 forms that have been identified and described as belonging to the 

 structure of Eozoon. The granules of serpentine are sometimes ar- 

 ranged quite regularly in concentric lines, but more commonly 

 appear irregularly scattered and varying indefinitely in form and size. 



Fig. 1 (PI. n), shows the arrangement of the serpentine in a piece 

 of the Chelmsford ophi-calcite or serpentine limestone. This and 

 the following figures were printed from plates copied by the electro- 

 type process from decalcified surfaces of the rock. In this specimen 

 the larger calcite spaces showed under the microscope a great abun- 



