b POST-TERTIARY LACUSTRINE SAND 



when the quartz, mica, and other matters which separate them 

 are removed, a mixture of a very remarkable kind, in which 

 a large number of forms are tolerably abundant, and a still 

 larger number are pretty frequent, while none are so pre- 

 dominant as we find them in recent gatherings, and a good 

 many are so scarce, that we have often to search long before 

 finding additional specimens, although with patience we 

 generally succeed in doing so. 



The peculiar constitution we have described renders a com- 

 plete study of this deposit a work of much time and labour. 

 I soon found that it was only by pursuing the minute and 

 systematic mode of search which I have described in my 

 account of the Mull deposit, that I could hope to determine 

 the species present in this one. I have found it, however, 

 advantageous, in consequence of the large number and relative 

 scarcity of new forms in the Glenshira sand, to adopt the plan 

 of marking any striking forms, or such as require to be ex- 

 amined, or are to be figured, when first observed. I find the 

 best way of marking is, after fixing on the form, to put on 

 the 2-3rd objective, and under that power to place one spot of 

 ink just above, but not on, the form. This is much more 

 rapidly and easily done than drawing a circle round it, and 

 it interferes much less with the remaining forms. A note is 

 kept of all the spots made on each slide, arranging them in a 

 certain order, according as they follow in the regular course 

 of sweeping the slides. By this means any marked form is 

 instantly recovered ; and I have been able to place in the 

 hands of Mr. West, in the course of one forenoon, a number 

 of new and striking forms so great, that without some such 

 method I could not have pointed them out, from their com- 

 parative scarcity, under a much longer time. 



It may be here mentioned, that, in studying a mixture like 

 the present, no examination, short of a thorough and minute 

 search, would suffice. Without this we should infallibly miss 

 a large proportion of the most interesting forms. To give 

 some idea of the necessity of this, I may state that I have 

 found it necessary to explore, minutely and repeatedly, 60 

 well-filled slides of this deposit, and that I have not yet ex- 

 hausted it, as even now 1 hardly ever search one of these 

 slides without observing something new or interesting pre- 

 viously overlooked. 



This is no doubt very laborious, but without labour nothing 

 can be well done, and in the present case the results have been 

 highly satisfactory. I have recognised upwards of 200 known 

 species, while a number remain that for the present I cannot 

 exactly name, for want of good figures ; and besides this, 1 

 have distinguished about 25, probably more, new and un- 



