220 



KNOWLEDGE. 



[October 1, 1898. 



degrees north, about tbatof the Shetlands and Christiania. 

 Could we procure a similar climate, and similar means of 

 precipitation, along the west side of the Leinster Chain, a 

 glacier as large as the Malaspina would cover all the 

 lowland area of Kildare, Carlow, and Queen's County. 

 Indeed, our highlands, as they now exist, would have 

 gone far, at the close of the Glacial epoch, to keep the 

 plain of Ireland full of ice. 



Whatever the cause, the means of precipitation were 

 actually provided ; but at last the modern epoch opened. 

 The sun shone on the ring of snow-peaks from Lough 

 Foyle to Galtymore, on the long moor of Leinster, and on 

 the white plateaux of the north ; but centuries may have 

 elapsed before the lowlands were free from the cold 

 burden thrust upon them. The ice of the plain was full 

 of intraglacial drift, shot into it by avalanches and land- 

 slides, or slowly incorporated with it by the glaciers 

 descending from the hills. As melting began, this 

 gravelly detritus would appear, capping, for instance, the 

 islands of Clew Bay, or streaming down as delta-formations 

 far out into the Irish Sea. Broad stratified deposits might 

 be formed by a union of marine and river action ; but in 



Fig. 4. -Stratification of S;,imI ;,t li.-i- of tlip Grppii Hill- Esker, 

 ( ... Ilul.lm. 



the interior of the country the deposits would be more 

 hummocky and isolated, and would often represent the 

 courses of the last subglacial streams. The plain of ice 

 might in time become reduced to separate patches, each 

 with its fringe of hillocks, piled up from intraglacial drift ; 

 and, where melting was slow and steady, true eskers might 

 remain, sinuous and steep-sided, as casts of the more 

 permanent waterways. For a long time, the torrential 

 flow would have kept such channels open ; so that the 

 eskers represent the final accumulations, due to failure of 

 the water-supply, and are younger than many of the 

 distributed gravels, which originated equally from the 

 intraglacial drift. 



Such appears at present to be the logical history of eskers, 

 like that of Lalrothery and Crumlin. The stratification 

 in the Green Hills of Co. Dublin is marked in the basal 

 sands, but is highly irregular in the gravels of the summit 

 (Figs. 3 and 4), and this is what might be expected from 

 the suggested conditions of formation, the material having 

 been washed down, at different times, with very different 



rates of flow. The freshness of the esker slopes, and the 

 preservation of the ridge-like form, may be paralleled by 

 the undisturbed outlines of the extinct scoria-cones of 

 Auvergne. In both cases, the porosity of the material 

 allows the water to sink through it, and a few channels here 

 and there alone mark the attack of exceptional storms.- 



We have, in conclusion, to go to the uplands of Tyrone 

 to see what a part the " esker- drift " may play in the 

 present conformation of the surface. Near Dunnamore, 

 for instance, we may see a giant esker running across 

 country, descending one side of the valley and climbing up 

 the opposite slope, with all the persistent air of the Great 

 Wall of China. In the hollow below us, the trend of which 

 is scorned by it, the esker is breached by the existing stream. 

 Clearly, its central part must have formed at one time the 

 barrier of a temporary lake. ^\'hen we ascend to the 

 moorland over against us, we find the gravel ridge lost in 

 a plexus of curving mounds, in the bays of which lakelets 

 lie gleaming in the western light. As the sun sinks, the 

 shafts pick out the soft green flanks of gravel domes, 

 sometimes isolated, sometimes clustered in all manner of 

 strange positions on the far hill-sides. Even on the high 

 spurs of Slieve Gallion, Lough Fea is bordered by them, as 

 if by the dihris of a landslide. We look back along our 

 grass-grown wall, the one side of which is now cold and 

 purple-grey, the other golden in the sunset. It stands out 

 before ua more sharply than ever, still more strange and 

 fascinating ; and we feel that we have a good deal yet to 

 learn with regard to the origin of eskers. 



THE SEA-SQUIRT. 



By E. Stenhouse, a.b.i.s., b.s. 



THE sea-squirt has such a curious organisation, and 

 passes through so strange a series of changes in 

 its development, that it and its allies have long 

 been regarded with more than usual interest by 

 naturalists. For the sea-squirt is a living example 

 of degeneracy, of structural degradation so complete that 

 until recently it was universally supposed to be a mollusc. 

 Its shape is roughly cyUndrical or ovoid ; its colour a dingy 

 grey ; and it lives attached by its base to a rock on the sea- 

 shore. At its free end there is a hole, commonly sur- 

 rounded by eight small lobes, and a little less than half- 

 way down the side of the body is another opening, with 

 six encircling lobes. The upper aperture is the mouth, 

 and it leads to the digestive tube, which consists of a 

 spacious pharynx immediately following the mouth, a gullet, 

 a stomach, and an intestine. Completely surrounding the 

 digestive tube, except along one line, where the pharynx is 

 fused with the body-wall, is a chamber called the atrium. 

 The atrium opens to the exterior at the lower of the two 

 external apertures, which is hence called the atrial opening. 

 If the Ascidian be carefully watched under natural con- 

 ditions, a current of water wUl be seen to continually enter 

 the mouth and leave by the atrial opening. If it be 

 touched the creature wiU suddenly send out a stream of 

 water from each opening, and its common name is derived 

 from this habit of squirting when irritated. The inflowing 

 current of water is doubly useful to the Ascidian. It not 

 only washes into the digestive canal the microscopic 

 organisms which constitute its food, but it also carries in 

 solution a store of fresh oxygen, which is just as necessary 

 for the healthy life of the animal as it is for our own well- 

 being. The region of the pharynx which is fused with 



* See Judd, "Volcanoes," p. l.")o; Lyell. "Principles of Geologv, " 

 Vol. II. (1833), p. 205. 



