164 THE MICROSCOPE. Nov. 



are going to prove, and you know you can swear to a 

 person's identity in a court of law without knowing his 

 name. Rounded or ovoid fluid inclusions without gas 

 bubbles are classified as regular. 



II. Acicular. These are fine needles, sometimes very 

 fine indeed; sometimes so fine that under the microscope 

 they appear as Tennyson might have put it, "as a finer 

 line in light.'" They are usually straight — a dead 

 straight — and I know of nothing that so nearly repre- 

 sents the technical definition of a line as "length with- 

 out breadth " as do some of these needles. Occasionally, 

 however, we see a zigzag one running right through a 

 crystal, and appearing to the eye very much the same as 

 the forked lightning of the "Old Masters." These need- 

 les are generally fairly equally distributed through the 

 grains, though, occasionally, they colonise in a particular 

 part of it. 



III. The spirit level kind hardly comes under observa- 

 tion at all. If they do, they are usually associated with 

 members of the regular group, under which the grain is, 

 in that case, classified. 



IV. Irregular inclusions are very indefinite in shape 

 and size. Some of them are empty, when they appear 

 nearly black, as if the grains were daubed over with 

 black ink spots. Some of them contain fluid with or 

 without a bubble of gas. They are often so small that 

 with ordinary powers of the microscope the grain ap- 

 pears inclusionless, and is so classified in the results. 

 This source of error is, however, of no importance. 



Now, you ask what is the meaning of all this. What 

 use is it to be put to ? My thesis — my contention is this 

 — that, by reason of these inclusions, we are enabled, 

 with very great certainty in many cases, to refer the in- 

 dividual grains to their parent rocks. Just in the same 

 way as a geologist well acquainted with the geology^of 

 a district may go to a pit of Boulder Clay, and picking 



